DE2552954A1 - DEVICE FOR MOISTURE MEASUREMENT OF SPATIAL EXTENSION SAMPLES - Google Patents

Description

509 Leverkusen. Bayerwerk

Device for moisture measurement of spatially extended samples

The invention relates to a device for measuring moisture of spatially extended samples, e.g. paper, wood, masonry, soil, etc. The measurement is based on microwave absorption of water. The device consists of a frequency-modulated microwave generator, the frequency deviation of which is set so that the complete resonance curve of both the empty cavity resonator and the cavity resonator exposed to the sample were swept over will. The frequency-modulated microwave generator feeds a resonator, the damping of which by the sample is a measure of the desired Humidity is. The resonator is a microwave detector for detecting the reflected or transmitted microwave signal downstream.

Moisture meters based on the principle of microwave absorption are known. In the German Offenlegungsschrift 2 340 130 a method is described in which tape-shaped samples (e.g. paper or film webs) are continuously monitored for their water content on the basis of microwave absorption will. The tape-shaped sample is continuously guided through the separating gap of a two-part resonator. The resonator is fed by a frequency-modulated microwave generator, the frequency deviation being chosen so that the complete Resonance curve of both the empty and the resonator filled with the sample is swept over. Through the rehearsal

Le A 16 608

709822 / 0B08

the resonator is detuned, so that the resonance frequency increases lower values. The change in the quality factor of the resonator is used as a measured variable for the water content. Either the signal reflected by the resonator or the transmission signal can be recorded.

This method has proven itself very well for measuring tape-shaped samples. A fundamental disadvantage of this arrangement is, however, that only those samples can be measured that can or can be passed through the resonator which are so small that they can be introduced into the resonator. The procedure is based on the principle of two-dimensional Samples or very small samples limited. From a technical point of view, however, the moisture measurement is also carried out on spatially extended Samples of considerable interest. The term "spatially extended" is to be understood here as meaning that the sample material has only one area for placing the measuring head, but otherwise has any spatial extent. Typical examples for this are wooden parts, chipboard, concrete walls, masonry, balls of fabric, color pastes, etc. Furthermore there is e.g. in the food industry the need for moisture measurements on powdery, paste-like or granular material. An example is the determination of the residual moisture in milk powder, powder coffee or tobacco. In these cases must the measuring head be designed so that it is surrounded by the product to be examined.

To date, no microwave measuring methods are known for such samples.

The invention is therefore based on the object, proceeding from the microwave measuring method mentioned at the beginning, for such a method To develop a suitable moisture meter for samples.

This object is achieved according to the invention in that the measuring head containing the resonator is used as a contact or immersion probe is formed and the resonator is made of a one-piece hollow

Le A 16 608 - 2 - »/

709822 / 050B

There is a space resonator that takes the place of an E or H field maximum one or more openings with a diameter of 0.5 K (K = microwave length) through which an electrical Stray field emerges in the sample placed on top or moved over it.

At first one could not expect that one would arrive at a usable microwave moisture measuring device in this way. Because of the breakthrough in the resonator wall can, on the one hand, leak microwave energy and the measurement sensitivity by reducing the quality factor as a result of radiation attenuation. On the other hand, by reflecting on the well-worn Radiation, standing waves mainly arise from moving objects in the vicinity, which are caused by a reaction on the measuring resonator reduce the measurement accuracy.

Contrary to expectations, however, the microwave moisture measuring devices according to the invention are relatively insensitive to these interferences and achieve a remarkably high measuring accuracy of the order of 3 % of the end value of the measuring range.

The resonator is preferably a rectangular resonator in which the iLjQ field type is stimulated, designed. The leaking electrical When coupled out via an E-field maximum in the resonator, the stray field has a practically completely symmetrical radial one Structure that has an anisotropy effect of microwave absorption in materials with directional structures, e.g. fiber bundles, Paper, chipboard, avoids. Alternatively, a cylindrical resonator with the EL ^ field type can be used, wherein the breakthrough is arranged at the location of an H-field maximum is.

An important improvement of the invention is that in the middle of the opening in the resonator parallel to his In the longitudinal direction, a wire-shaped conductor is arranged, the length of which corresponds approximately to the diameter of the opening. In this way one can achieve that the stray field passing through the breakthrough is better concentrated in the sample. this may be necessary if the sample is e.g. from a plate

Le A 16 608 - 3 -

7G9822 / 0S0S

exists and the stray field would reach through the plate into the free space without additional tools. The damping of the resonator would then not only be caused by the dielectric losses in the sample but also by radiation into the free space.

Instead of one wire, several wires or several adjacent bores of smaller diameter can also be made in the resonator wall be used. This causes the stray field to subside within a very short distance outside the Resonators.

In order to prevent sample parts from falling into the resonator, the breakthrough is expediently made with a Teflon disk (polytetrafluoroethylene) covered. For the same reason one can also think of the resonator entirely with polytetrafluoroethylene as a dielectric to fill. This material is practically lossless at 10 gigahertz. The last two modifications mentioned of the invention are important when the measuring device is used as an immersion probe for measuring pasty or powdery material is trained.

The real problem of the new microwave measuring device lies in the fulfillment of the two following contradicting one another Requirements:

1. The depth of microwave penetration into the sample must be large enough in order to achieve a sufficiently high measurement sensitivity and to capture a representative sample volume.

2. On the other hand, radiation of the microwaves into the room must be avoided.

Practice has "shown that these two requirements can be mutually agreed. The new device has proven itself to be excellent for precise moisture measurement in the above-mentioned materials. A measurement accuracy of approx. 3 % of the end value of the measuring range can be regarded as sufficient. The sensitivity is reflected Expect so high that even the residual moisture area can be painted over

Le A 16 608 - 4 -

7098 2 2/0 506

Understood moisture values in the range from about 0.1 to 10% by weight. If the measuring head is designed as an immersion or penetration probe, the moisture from powdery or pasty material can be determined. Furthermore, the measurement of soil moisture is an important area of application for this Measuring head type. For this purpose, e.g. the microwave resonator designed as a penetration probe is placed inside a guide tube sunk in the ground.

In the following, exemplary embodiments of the invention are described in more detail with reference to a drawing.

Figure 1 shows the basic arrangement of resonator and sample,

FIG. 2 the field distribution in the vicinity of the breakthrough outside the resonator,

FIG. 3 shows the basic circuit diagram of the measuring arrangement in transmission, FIG. 4 the basic circuit diagram of the measuring arrangement in reflection,

FIG. 5 shows the resonance curves of the empty resonator and the resonator damped and detuned by the applied sample,

FIG. 6 shows an embodiment in which the resonator is used as a penetration probe is trained,

FIG. 7 shows the diagram of the embodiment of a measuring resonator built into a guide roller for strip-shaped material to be measured.

Since the maximum of the microwave absorption for the free water

10

molecule is around 10 Hz, the frequency of the microwave generator is set in this area in order to achieve the greatest possible measuring effect. Commercially available microwave components can then be used use for the X-band. From Fig. 1, the measuring principle can be seen. The resonator 1 is a one-piece rectangular resonator, "in which the H ^^ field type is excited. It is via a Iris coupler 2 connected to the microwave generator. The output is via a similar iris coupler 3 to the microwave detector coupled. In the top surface of the resonator, in the middle of the broad side, there is predominantly electrical

Le A 16 608 - 5 -

709822 / OBOS

tric field strength (E-field maximum) a circular breakthrough 4 with a diameter of 10 mm. The breakthrough can, however, also advantageously be designed in such a way that primarily the magnetic field component emerges. With a microwave length of 3 cm, the condition is met that the breakthrough should be <^ - . The sample to be examined, in this case a large plate 5, is passed over the resonator 1 or the resonator 1 is placed against a wall. With this dimensioning, the stray field enters the sample approximately ~ - deep. This prevents radiation attenuation. In the unloaded state, the resonator 1 achieves a quality of approx. §00. This enables a sensitive residual moisture measurement of paper, wood, masonry, concrete etc. The prerequisite for an accurate moisture measurement is the smallest possible air gap between the contact surface of the resonator and the surface of the test object. In the case of greater surface roughness and greater thicknesses of the test objects, it can be useful to design the apparatus for a lower frequency, for example 2 GHz.

From Figure 2, the electric field distribution is in the vicinity of the opening 4 can be seen when a wire-shaped conductor 6 is additionally arranged in the longitudinal direction of the pipe in the center of the opening will. The length of the piece of wire is usually equal to the diameter of the opening 4.

The resulting electrical stray field results from a superposition of the emerging Ε-field, the H.Q-field type and on the other hand through the fields of the wall currents flowing in the wire-shaped conductor. The wire-shaped conductor causes a better concentration of the stray field on the sample area. The breakthrough need not necessarily be circular. Breakthroughs of other shapes, e.g. Rectangular shape, are electrically equivalent, but have the disadvantage of being mechanically more difficult to manufacture to be. Instead of one opening with a larger diameter, several with a smaller diameter can also be provided if the measurement is to extend over a larger area of the sample. Of the possibilities of an additional Field concentration will be used when it is found that the change in quality of the cavity resonator is not alone due to the dielectric losses of the sample, but also a contribution from radiation attenuation is present.

709822 ^ 0506.

FIG. 3 shows a block diagram for the one indicated in FIG Transmission arrangement. As already described, the resonator 1 is connected to the microwave generator via the iris coupler 2 7 and coupled to the detector 12 on the output side. The degree of coupling depends on the diameter of the iris bores and influences the quality factor of the resonator. In this way, the measurement sensitivity can be determined via the geometry of the iris coupler 2 and 3 set. In order to avoid interference from reflected signals, the microwave line leading to the resonator 1 is used connected via a one-way line.

In the following, the microwave measuring arrangement is illustrated using the basic circuit diagrams (Fig. 3 and Fig. 4) explained. Figure 3 shows a transmission circuit. The transmission resonator 1 is via a variable damping element 9 from the microwave oscillator 7 fed. A varactor-modulated Gunn diode oscillator is used as the microwave oscillator 7, which by a sawtooth generator 10 is frequency modulated linearly in time. The microwave power output is swept across the whole Frequency range almost constant. The modulation frequency is approx. 2 KHz. Choosing a relatively high modulation frequency has the "advantage that rapid changes in measured values can also be recorded. This is, for example, in the case of sudden changes Changes in humidity along a plate-shaped sample important. In practice, a modulation frequency between 50 Hz and 5 KHZ is sufficient. The frequency deviation of 150 MHz at 9 GHz carrier frequency, is dimensioned in such a way that the resonance curve of the empty resonator and the resonator loaded with the test object with certainty be painted over (see Fig. 5). The microwave power of the oscillator 7 is transmitted to the reference detector via a directional coupler 13 11 and fed via the variable attenuator 9 to the microwave resonator 1 connected in transmission. The transmission signal (Resonance curve of the resonator) delivers after rectification of the microwave the measuring detector 12. It has the one shown in FIG shown form. With increasing water content, the peak amplitude U ^ decreases, at the same time there is a shift in the Resonance frequency connected to lower frequencies. the

Le A 16 608 - 7 -

709822/0506

'/ to .

The peak amplitudes of the detector signal for the resonator with and without a sample are U ^ and U _Q. The reference detector delivers the signal U _p · Uj and thus U-] - Ur is a monotonic radio

it

tion of the quality factor and thus the material moisture. A reference signal is branched off with the directional coupler 13 and rectified by the reference detector 11. Reference detector 11 and Measurement detectors 12 are peak voltage rectifiers. The rectified peak voltage values U ^ and Ur are then dem Differential amplifier 14 supplied. The display takes place with a measuring instrument 15 or a recording device. The bridge circuit (Comparison branch consisting of directional coupler 13 and reference detector 11) has the advantage that fluctuations in the ambient temperature or the power of the microwave oscillator 7 have practically no influence on the measurement accuracy.

Instead of a transmission arrangement, a reflection circuit (FIG. 4) can of course also be used. The reflection resonator 1 is preceded by a circulator 16 which forwards the signal reflected by the resonator to the measuring detector. For small changes in attenuation due to the dielectric loss of the water contained in the sample, the water content c " _q in the form

^c H ₂ 0 = ^{k £} "= ^f Ι / Φ] ^with * Φ = ¹ / ^Q 1 - ¹ / ^Q o

can be determined (ASTM standards 13 (1964) 465, W. Eckhardt et al, Zs. angew. Physik 6 (1954) 236). Mean:

£ ": The imaginary part of the DC of water CLj: The quality of the resonator with sample Q: The quality of the resonator without a sample.

The water content is therefore a clear function f of the resonator quality. This function still has to be calibrated for absolute measurements. For this purpose, samples with known water content are used brought to the top surface of the resonator.

Le A 16 608 - 8 -

709822/0506

The resonator designed as a penetration probe according to FIG. 6 is used to determine the moisture content of powdery or paste-like material. In addition, it can be used to determine the soil moisture in a certain layer of soil. To this end, will the penetration probe, e.g. in a polytetrafluoroethylene tube 17 lowered into the ground. The measuring head is formed here by a cylindrical resonator 18 in which the H ^^^ wave is excited is. The hole couplers 2 and 3 for coupling and decoupling the microwave energy are arranged here next to one another. The breakthrough 4 is located in the lateral surface of the resonator 18. When measuring powdery materials, the resonator closed with a PTFE disk (polytetrafluorethylene) so that no sample material gets into the resonator. For the same The resonator can also be completely filled with PTFE. The dielectric losses from this material are in the frequency range of 10 gigahertz negligibly small.

The installation of a stray field probe in a product guide Roller 19 for moisture measurement of flat, continuously moving material (see Fig. 7a) e.g. paper or fabric webs, offers advantages over the use of two-part resonators (see e.g. DT-OS 2 340 130). Since the measurement is only from one side takes place, there is no measuring gap that limits the product shape. As a result, the measuring frequency can be used regardless of the measuring gap width can be freely chosen. In the case of the two-part resonator, the measuring gap width and the frequency cannot be freely selected, since the measuring gap width must not fall below ^ to avoid radiation. To measure high and medium water contents one becomes Achieving a calibration curve that is as linear as possible and, for price reasons, e.g. work at 2 to 9 GHz while using the sensitive Residual moisture measurement and to eliminate competing absorptions (e.g. other dipole molecules, semiconductors) frequencies up to about 22 GHz used. Due to the limited penetration depth of the measuring radiation, the use of the scatter field probe also offers the advantage of that provided that the product is sufficiently thick, fluctuations in thickness do not affect the measurement result. Through the installation the stray field probe in a guide roller according to FIG. 7 results in the possibility of alternating registration of the

Le A 16 608 - 9 -

709822/0506

Measured value (when the product rests on the stray field outlet) and the zero value when the stray field opening is released by the product after further rotation of the roller 19 (see Fig. 7). The angle of wrap of the measuring roller 19 should be approximately 90 for this purpose. The alternately registered measured values U ^ and U _Q can be used for the exact characterization of the humidity value U-IL as described above and for standardizing the measuring arrangement, so that a reference detector can be dispensed with. The stray field probe is installed in the measuring roller (see Fig. 7b) in such a way that the breakthrough 4 in the resonator wall which emits the stray field is flush with the surface of the roller. is easily possible.

The entire microwave arrangement is expediently located inside the roller, so that only direct current signals have to be transmitted via sliding contacts 21. However, the microwave signal can also be supplied via rotary couplings. The metrological advantage of this arrangement is that the measurement and zero signals are recorded alternately. To separate the measurement and zero signals, an amplifier connected as an electrical switch is used, which is controlled by the angle encoder in the following way: During the time t ^ the signal U ₁ and during the time t the signal U _{Q is} detected, t ^ is the time during which the openings 4 are covered by the web 23 and t _Q the time during which they are free, t ^ and t _Q are assigned to corresponding switching positions of the angle encoder 22.

Le A 16 608 - 10 -

709822/050 $

A3

Blank page

Claims (8)

Patent claims: Device for measuring the moisture of spatially extended samples, e.g. paper, wood, masonry, color pastes, soil, consisting of a frequency-modulated microwave generator that feeds a resonator, whose damping is caused by the sample is a measure of the humidity sought, the frequency deviation of the microwave generator being selected so that the complete Resonance curve of both the empty and the resonator charged with the sample is swept over and the resonator a microwave detector for the microwave signal reflected or transmitted by the resonator is connected downstream, characterized in that the measuring head containing the resonator (1) is designed as a contact or immersion probe and the resonator (1) consists of a one-piece cavity resonator, which is at the point of maximum electrical or magnetic Field strength (E or H field maximum) a breakthrough with a diameter of 0 <0.5 X (S. = microwave length), through which a stray electrical field emerges into the sample (5) placed on or passed over it. 2. Apparatus according to claim 1, characterized in that the resonator (1) is a rectangular resonator with the H ₁₀ field type. 3. Apparatus according to claim 1, characterized in that the resonator is a cylindrical resonator with the H ₁₁ field type. 4. Apparatus according to claim 1 to 3, characterized in that in the middle of the opening (4) parallel to the longitudinal direction of the resonator a wire-shaped conductor (6) is arranged, the length of which corresponds to the diameter of the opening (4) matches. 5. Device according to claim 1 to 2, characterized in that that the opening (4) is covered with a wire mesh. Le A 16 608 - 11 - 709822/0506 6. Apparatus according to claim 1 to 5, characterized in that the opening (4) with a polytetrafluoroethylene disc or foil is covered. 7. Device according to Claims 1 to 5, characterized in that the resonator (1) uses polytetrafluoroethylene as the dielectric is filled. 8. Apparatus according to claim 1 to 7, characterized in that the resonator is part of a guide roller (19) for an areal Forms material to be measured (23), the opening (4) being integrated into the roller surface for the exit of the stray field and the entire microwave electronics (10, 7, 9, 13, 11, 12) is housed within the roller (19). 9. Device according to claim 8, characterized in that a switch amplifier controlled by a rotary encoder (22) is used for the separate display of the signals from the detector U _{Q and IL which are dependent on the angle of rotation.} Le A 16 608 - 12 - 709822/0506